The present invention relates to flexible couplings for transmitting rotation between angularly misaligned shafts. More particularly, the invention relates to articulated couplings.
The invention may be most suitably used in screw-type downhole motors for drilling boreholes as a unit for connecting a rotor moving in a gyratory path with a shaft of a supporting unit to which the drill bit is connected.
The invention may also be used in turbo-drills for controlled directional drilling, designed for changing the direction of a borehole in the process of drilling.
The invention may be used in other machines, for example, in screw-type pumps.
In the screw-type downhole motors and screw-type pumps, use is made of two articulated couplings connected one after the other, while in the turbo-drills for controlled directional drilling, use is made of one articulated coupling.
The use of articulated units in the downhole mechanisms is associated with transmission by these units of torque and axial thrust of great magnitude from a driving member to a driven member such as, for example, a drill bit. In screw-type downhole motors with multiple-start driving members torque amounts to 4000-5000 Nm while axial thrust amounts to 100-150 kN). Another feature in the use of articulated units resides in stringent limits imposed on their diametral dimensions because of the need for placing the downhole motor in a borehole.
Known in the prior art is a flexible coupling for a screw-type downhole motor, disclosed in U.S. Pat. No. 3,260,069. This coupling comprises upper and lower connectors with which are associated tubular segments provided with shaped lobes and lobe spaces due to which the lobe of one segment fits the lobe space of the adjacent segment with clearances. Disposed in the center hollow portion of the segments are spacing elements, each of which has a spherical socket in both end surfaces for accommodating a ball. From the outside the flexible coupling is enclosed in a boot of elastic material for retaining the lubricant in the flexible coupling. The upper and lower connectors are associated respectively with the rotor and the output shaft of a motor.
Transmission of torque by the flexible coupling is accomplished due to interaction of the lobes of the adjacent tubular segments, while axial thrust is transmitted through the spacing elements and the balls fitted in the sockets thereof.
Due to the presence of clearances between the lobes of the tubular segments the latter can be angularly displaced relative to one another, with the result that the coupling acquires the needed flexibility. However, when the flexible coupling is in operation the lobes of the tubular segments continuously slide one over another which under conditions of high unit loads at places of contact brings about friction losses and wear of these contacting surfaces.
In addition, sliding friction and wear are caused when axial thrust is transmitted through the spacing elements and the balls, as the surfaces of these elements are also engaged in sliding motion.
Also known in the prior art is an articulated coupling used for connecting the rotor and the output shaft of a screw-type downhole motor (U.S. Pat. No. 3,357,208), comprising an external half-coupling provided with internal teeth, an internal half-coupling provided with external teeth interacting with the internal teeth of the external half-coupling, a tube connecting the internal half-couplings with each other, a means for transmission of axial thrust, and seals. The means for transmission of axial thrust incorporates thrust bars secured in the external and internal half-couplings so that the point of contact of the thrust bars lies in the plane passing through the middle of the teeth.
Angular misalignment of the coupling is accomplished due to the backlash in the gearing of the half-couplings, through which transmission of torque is effected. Due to a continuous angular precessional motor of the coupling, transmission of torque through the surfaces of the teeth of the half-couplings is accomplished by their intensive sliding and wear of the contacting surfaces.
Besides, when the gearing operates wiht angularity, there exists an obvious nonuniform distribution of load over the teeth of the half-couplings, since the teeth which lie in a plane perpendicular to the plane of angularity are the most loaded. These teeth have an edge contact causing distortion and breakage thereof.
Another articulated coupling for downhole fluid motors has been known heretofore in the art (cf. U.S. Pat. No. 4,157,022), comprising two heads with yokes, and a center cross disposed therebetween. Pins pass through openings of the yokes and the center cross in a mutually perpendicular direction. One of the pins is solid and the other is constituted of two portions.
Transmission of torque is effected due to interaction of internal side surfaces of the yoke with external surfaces of the center cross and also due to interaction of the pins with surfaces of the openings in the yokes and the center cross. The aforedescribed coupling has a lubrication system with a lubricator which balances the pressure of lubricant in the joints and the pressure of fluid surrounding the joints.
A disadvantage of this articulated coupling resides in the fact that transmission of torque and axial thrust is effected through the surfaces of the elements engaged in a mutual relative sliding motion causing an excessive wear and reducing durability of the coupling.
There is also known an articulated coupling described in the book "Single-Screw Pumps" by Krylov A.V., Moscow, Publishing House "Gostoptekhizdat", 1962, p. 58, FIG. 41, in Russian. This coupling comprises two cylindrical half-couplings, a cage surrounding the half-couplings, and rollers. Formed in the external surfaces of the half-couplings and in the internal surface of the cage are longitudinal slots of cylindrical section, the axes thereof being parallel to the axes of the half-couplings and the cage. Cylindrical rollers are disposed in said slots between the half-couplings and the cage. Spherical sockets are formed in the end faces of the half-couplings and accomodate a thrust member whose ends are also spherical. Angular misalignment of the coupling results from the presence of clearances between the surfaces of walls of the slots and the rollers. Transmission of torque is effected due to interaction of the surfaces of the slots in the half-couplings and the cage with the rollers accommodated in the slots. Axial thrust is transmitted from one half-coupling to the other through the thrust member.
The main disadvantage, that is, a relative slip of the interacting surfaces, of the aforedescribed couplings is also characteristic of this articulated coupling. During the oscillatory (precessional) motion of the articulated coupling, the surfaces of the slots of the half-coupling slide over the surfaces of the rollers along the axes thereof, the path of sliding being defined by the magnitude of angular misalignment of the coupling and the distance from the axis of the half-coupling to the surface of the roller.
Another disadvantage of the known articulated coupling resides in the fact that under the action of a couple of oppositely directed forces exerted on the side of the driving and driven half-couplings, the rollers tend to turn so that their axes are angularly misaligned in relation to the axes of the slots in the half-couplings and the cage. The edge contact of the rollers with the surfaces of the slots resulting from this condition leads to rapid wear or breakage of the rollers. Due to these disadvantages the torque-transmitting capacity of the articulated coupling is substantially limited.
Besides, when said articulated coupling is in operation wedging out forces appear on the case, since the direction of forces acting at the place of contact of the roller with the surfaces of the slots is not coincident with the direction of the perpendicular to the radius, drawn to the point of contact.